1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for horn antennas, and more particularly to horn antennas which can operate in multiple frequency bands.
2. Description of the Related Art
Conventional electromagnetic waveguides and horn antennas are well known in the art. A waveguide is a transmission line structure that is commonly used for microwave signal transmission. A horn antenna (horn) is a particular type of waveguide which is optimized for wireless propagation and reception of RF signals through free space. Horns typically have a conical or pyramidal shape. In a common configuration, a narrow end of the horn is operatively connected to one end of a feed structure, which itself includes a waveguide. A feed probe is usually installed at an end of the feed structure which is opposite of the end of the feed structure that is joined with the horn. The feed probe couples the horn to transmit and/or receive circuitry and is typically optimized for operation in the particular frequency band for which the horn is designed to operate.
The horn and feed structure typically include a material medium that confines and guides propagating electromagnetic waves. In a horn or feed structure, as with most other waveguides, a “mode” is one of the various possible patterns of propagating electromagnetic fields. Each mode is characterized by frequency, polarization, electric field strength, and magnetic field strength. Each horn configuration can form different transverse electric and transverse magnetic modes. Since horns are generally designed to have a static geometry, the operational frequency and bandwidth of conventional horns are limited.
To overcome the frequency and bandwidth limitations of horns, International Patent Application No. PCT/GB92/01173 assigned to Loughborough University of Technology (Loughborough) proposes that a frequency selective surface (FSS) can be used within a horn to influence the frequency response. An FSS is typically provided in one of two arrangements. In a first arrangement, two or more layers of conductive elements are separated by a dielectric substrate. The elements are selected to resonate at a particular frequency at which the FSS will become reflective. The distance between the element layers is selected to create a bandpass condition at a fundamental frequency at which the FSS becomes transparent and passes a signal. The FSS also can pass harmonics of the fundamental frequency. For example, if the fundamental frequency is 10 GHz, the FSS can pass 20 GHz, 30 GHz, 40 GHz, and so on.
Alternatively, FSS elements can be apertures in a conductive surface. The dimensions of the apertures can be selected so that the apertures resonate at a particular frequency. In this arrangement, the FSS elements pass signals propagating at the resonant frequency. Any other electromagnetic waves incident on the FSS surface are reflected from the surface. In a multi-band horn, the FSS can form a second horn within a first horn wherein the second horn and the first horn are tuned to different frequencies.
Hence, it would be desirable for a multi-band horn antenna to have multiple feed probes, with at least one probe being optimized for the operational frequency of each horn within the multi-band antenna. However, the coaxial nature of the feed structures in the multi-band horn antenna proposed by Loughborough prevents multiple feed probes from being incorporated into the multi-band horn antenna in a conventional fashion. Accordingly, there exists a need for a feed structure having multiple feed probes which can be used with a multi-band horn antenna having FSS's.